4. The Micro-instruction

If the Control Store is the "recipe book" for the CPU, then a micro-instruction is a single "line" in that recipe. It is the most fundamental unit of control in a microprogrammed CPU, defining the exact set of hardware operations that will occur in a single clock cycle.

• 4.1 Definition A micro-instruction is a single word fetched from the Control Store. Its primary function is to specify the control signals to be generated for one clock tick. Each micro-instruction is held in the Micro-instruction Register (uIR), and the bits of this register are used—either directly or after decoding—to activate or deactivate every control line in the CPU's datapath [1].

• 4.2 Format and Fields While the exact layout varies, a micro-instruction is typically divided into two primary types of fields [2].

  • 4.2.1 Control Field: This is the main part of the micro-instruction. It contains the bits that directly control the datapath components. For example, a "1" in a specific bit position might enable a register to output its value to the main bus (like RAO), while a "0" keeps it disabled. Another set of bits might specify the operation for the ALU (e.g., SUB=1).

  • 4.2.2 Sequencing Field (Next Address Field): This field doesn't control the datapath; it controls the Micro-Sequencer itself. These bits provide the sequencer with information to determine the address of the next micro-instruction. This field might contain:

    • A specific "next address" to jump to.
    • A "branch condition" code (like SEQ_DECODE or SEQ_JUMP_ON_ZERO) that tells the sequencer how to find the next address by checking opcodes or status flags [3].

• 4.3 Horizontal vs. Vertical Microprogramming The "Control Field" can be designed in two primary ways, which presents a classic trade-off between speed and memory efficiency [4].

  • 4.3.1 Horizontal Microprogramming: This is a "decoded" or "unencoded" approach. The micro-instruction is very wide, and each bit in the control field corresponds directly to a single control line. If the CPU has 60 control signals, the control field is 60 bits wide.

    • Pros: It is extremely fast. No additional decoding logic is needed; the bits from the uIR can be used directly. It also allows for high parallelism, as many signals can be activated in the same cycle.
    • Cons: It is very inefficient. A micro-instruction that only activates 2 signals (e.g., RAO and RBI) still requires the full 60 bits, wasting space in the Control Store.

  • 4.3.2 Vertical Microprogramming: This is an "encoded" approach. Instead of one bit per signal, groups of signals are encoded into fields. For example, if an ALU has 8 possible operations (ADD, SUB, AND, OR, etc.), a 3-bit field (2^3 = 8) can be used to select which operation to perform.

    • Pros: It is highly efficient and saves a significant amount of space in the Control Store. The micro-instructions are much narrower.
    • Cons: It is slower. The encoded fields (like the 3-bit ALU field) must be passed through an external decoder circuit to generate the final control signals, adding a layer of gate delay.

  In practice, most modern designs are a hybrid, using vertical encoding for mutually exclusive signals (like ALU operations) and horizontal bits for independent signals (like Memory_Write).